1. Field of the Invention
The present invention relates to an image compressing apparatus for compression-coding image data.
2. Related Background Art
JPEG (Joint Photographic Expert Group) algorithms are standardized as an international standard coding system for color still images.
The JPEG algorithms are roughly classified into DCT (Discrete Cosine Transform) as an irreversible coding system and DPCM (Differential Pulse Code Modulation) which is a reversible coding system capable of loss-less compression. Of these systems, one category of the DCT system, called a base-line process, has found widespread use as a de-facto-standard in both hardware (digital cameras) and software (e.g., viewers).
In performing conventional compression coding, some users need loss-less (reversible coding) compression. In the loss-less coding, data does not deteriorate however many times compression and expansion are repeated. However, if the DPCM system is used as the loss-less compression, an image compression and expansion apparatus must support both the DCT system and the DPCM system. This increases the hardware scale. Also, the data cannot be reproduced on personal computers because the loss-less compression is not supported by viewers.
If a quantization or subsampling step is omitted from the DCT system, it is possible to obtain compressed data close to loss-less compressed data, which cannot be distinguished from original images with the naked eye. The amount of this compressed data is larger than when quantization or subsampling is performed, and so a pipeline stall readily takes place in a compression apparatus. This extremely lowers the processing speed.